1. Field of the Invention
This invention relates to a radiation image read-out apparatus which obtains an image signal by reading out a radiation image of an object from a recording medium, such as a stimulable phosphor sheet, on which the radiation image has been recorded, and carrying out image processing of the image signal.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image to obtain an image signal, carrying out appropriate image processing of the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193an X-ray image is recorded on an X-ray film having a small gamma value designed for the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is image-processed and then used when the X-ray image is reproduced as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality and exhibiting such characteristics as high contrast, high sharpness, high graininess or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Patent Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is used when the radiation image of the object is reproduced as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
A radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using silver halide in that the amount of light emitted by the stimulable phosphor sheet is proportional to the energy intensity of the radiation, to which the stimulable phosphor sheet is exposed when an image is recorded thereon, and the energy intensity of said radiation may be selected from a very wide range (latitude) of radiation energy intensities. If an appropriate read-out gain is selected and used when the light emitted by said stimulable phosphor sheet is being detected, a desirable density can be obtained in the finally reproduced visible image regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed.
In order to assure that the conditions under which the image signal is read out are appropriate for the radiation dose to which the stimulable phosphor sheet or the like was exposed, the aforesaid radiation image recording and reproducing system may be constituted such that a preliminary read-out operation is carried out in which the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and the radiation image stored on the stimulable phosphor sheet is thus approximately ascertained. A preliminary read-out image signal is obtained for the preliminary read-out operation and is then analyzed. Thereafter, a final read-out operation is carried out. The conditions under which the radiation image is read out are determined on the basis of the results of an analysis of the preliminary read-out image signal. The stimulable phosphor sheet is scanned with a light beam having a comparatively high energy level, and an image signal is obtained which will be used during the reproduction of a visible image.
The term "read-out condition" as used herein means a group of conditions affecting the relationship between the amount of light emitted by the stimulable phosphor sheet during image read-out and the output of a read-out means. For example, the term "read-out condition" may refer to a read-out gain and a scale factor which defines the relationship between the input to the read-out means and the output therefrom, or the energy intensity of the stimulating rays used when the radiation image is read-out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the light beam, i.e. has a sensitivity which depends on the wavelength of the light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths or different intensities may be used. The intensity of a light beam may be changed by a laser beam source, or the like, or by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the energy level of a light beam also changes when the diameter of the light beam is changed, i.e. the scanning density is changed, or when the speed with which the light beam scans the stimulable phosphor sheet is changed.
Regardless of whether a preliminary read-out operation is or is not carried out, it has also been proposed to adjust the conditions under which an image signal (or a preliminary read-out image signal) is processed on the basis of the results of an analysis of the image signal or the preliminary read-out image signal. This proposed method is applicable both when a radiation image is recorded on a recording medium such as a conventional X-ray film and when a radiation image is recorded on a stimulable phosphor sheet.
In general, an operation (hereinafter referred to as EDR) which calculates the read-out condition and/or the image processing condition on the basis of an image signal (including a preliminary read-out image signal) is performed by an algorithm. The algorithm is designed on the basis of results obtained from the statistical processing of a large number of radiation images. However, in cases where the image of a large foreign substance (for example, a lead protector used for blocking radiation) is included in the radiation image of an object or a special type of image recording is carried out, an EDR often cannot be carried out accurately. In such cases, the final read-out operation cannot be carried out with an appropriate read-out condition, and/or an appropriate type of image processing is not performed. As a result, when the image signal, which was obtained from the final read-out operation and then processed, is used to reproduce a visible image, the visible image will have a density and contrast which make it unsuitable for viewing. In the worst case, the image of the object must be rerecorded. In cases where the object is a human body, the radiation dose to the human body is doubled when image recording is repeated. This problem should be avoided because radiation is harmful to the human body.
Examples of cases where the aforesaid problems arise will be described hereinbelow.
One of the characteristics of a recorded image which should be considered when selecting the algorithms for an EDR is that unnecessary portions of an object may be recorded on a recording medium when scattered radiation impinges upon those portions. Also, radiation may impinge directly upon a portion of a recording medium without being passed through or reflected by an object. In this manner, an image signal picks up unnecessary components which must be removed in order to obtain an image signal representing only the desired portions of a radiation image.
FIGS. 4A and 4B are graphs showing probability density functions of preliminary read-out image signals SP detected by preliminary read-out operations carried out on two stimulable phosphor sheets.
FIG. 4A shows an example of the probability density function of a preliminary read-out image signal SP detected from a radiation image for which an EDR is suitable which is of the type accounting for most (for example, 99.5%) radiation images.
With reference to FIG. 4A, the values of the preliminary read-out image signal SP which were obtained by detecting the light emitted by a stimulable phosphor sheet during a preliminary read-out operation and which are proportional to the amount of light emitted are plotted on the horizontal axis, which has a logarithmic scale. The relative frequency of occurence of the values of the preliminary read-out image signal SP is plotted on the vertical axis at the upper part of the graph, and the values of the image signal obtained during the final read-out operation are plotted on a logarithmic scale on the vertical axis at the lower part of the graph. In this case, the probability density function of the preliminary read-out image signal SP is composed of projecting portions A, B and C, and it is assumed that the projecting portion B corresponds to the part of a radiation image which it is necessary to reproduce. By way of example, in order to find the projecting portion B, the values of the probability density function are compared to a predetermined threshold value T, starting with the value of the function at the minimum value SL of the preliminary read-out image signal SP and working along the direction of increase of the image signal values, i.e. along the chained line. When the probability function crosses through the threshold value T, a calculation is made to find out whether the function is rising or falling. In this manner, a second rising point "a" and a second falling point "b" are found. The maximum and minimum values of the preliminary read-out image signal at the points "b" and "a" are denoted by Smax and Smin, respectively. The read-out condition for the final read-out is set so that during the final read-out operation the image information represented by the emitted light signal for values of the emitted light falling within the range of Smax to Smin is reproduced accurately. Specifically, the read-out condition for the final read-out is set so that Smax and Smin of the preliminary read-out image signal SP are detected respectively as the maximum image signal value Qmax and the minimum image signal value Qmin in the final read-out. The maximum image signal value Qmax and the minimum image signal value Qmin in turn correspond respectively to the maximum density Dmax and the minimum density Dmin within the predetermined correct density range of the visible image ultimately reproduced. More specifically, the read-out condition for the final read-out is set so that during the final read-out operation the image information represented by values of the emitted light signal falling within the range of Smax to Smin is detected as an image signal with values lying on the straight line G shown in FIG. 4A.
In the manner described above, for most of the radiation images, the read-out condition for the final read-out can be adjusted appropriately. However, in cases where, for example, an image of a foreign substance is included in a radiation image, correct read-out conditions cannot be determined with this method. One such case will be described hereinbelow.
FIG. 4B shows the probability density function of a preliminary read-out image signal SP' detected from a radiation image of an object approximate to the object, the radiation image of which yielded the probability density function shown in FIG. 4A. In the case of both FIGS. 4A and 4B, the radiation images of the objects (by way of example, the chest of a human body) were recorded by under the same image recording conditions, i.e. the characteristics of the recorded images were the same.
When the probability density function shown in FIG. 4B is compared with that shown in FIG. 4A, projecting portions B' and C' approximate the projecting portions B and C, respectively. However, a projecting portion A' differs from the projecting portion A, in that it is divided into two projecting portions, A1' and A2'.
When the method described above is applied to the probability density function shown in FIG. 4B, the values of the probability density function are compared to the predetermined threshold value T. Starting from the value of the function corresponding to the minimum value SL' of the preliminary read-out image signal SP', whenever the value of the function crosses over the threshold value T, a calculation is made as to whether the function is rising or falling. In this manner, a second rising point a' and a second falling point b' are found. However, the range of the preliminary read-out image signal SP' between the points a' and b' is different and far apart from the range (of the projecting portion B') corresponding to the parts of the radiation image, which it is necessary to reproduce. If the final read-out operation is carried out so that during the final read-out the image information represented by an emitted light signal with values falling within the range between the points a' and b' is detected as an image signal with values lying on a straight line G', the image signal thus obtained will not contain the necessary image information, and cannot yield a useful visible image. In such cases, the recording of the radiation image of the object must be repeated.
Besides the extreme case described above, an inaccurate EDR deteriorates the image quality of a reproduced visible image.